|  | 4.5 | Layer 3 Design
| 4.5.1 | Using routers as the basis for layer 3 network design
As shown in the Figure, Layer 3 (the network layer) devices, such as routers, can be used to create unique LAN segments and allow communication between segments based on Layer 3 addressing, such as IP addressing. Implementation of Layer 3 devices, such as routers, allows for segmentation of the LAN into unique physical and logical networks. Routers also allow for connectivity to wide-area networks (WANs), such as the Internet. Layer 3 routing determines traffic flow between unique physical network segments based on Layer 3 addressing, such as IP network and subnet. The router is one of the most powerful devices in the network topology. As you have learned, a router forwards data packets based on destination addresses. A router does not forward LAN-based broadcasts such as ARP requests. Therefore, the router interface is considered the entry and exit point of a broadcast domain and stops broadcasts from reaching other LAN segments.
| Threaded Case Study
Washington Project: Layer 3 Design Goals
The following are Layer 3 LAN topology design goals for the your site:

- Build a path between LAN segments that will filter the flow of data packets.
- Isolate ARP broadcasts.
- Isolate collisions between segments.
- Filter Layer 4 services between segments.